The present invention relates to a switch device used for a laser device, in which a number of high speed solid-state switch elements, such as field effect transistors (FETs), are simultaneously turned on and off, and more particularly to a switch device used as a high voltage switch for a discharge excited laser device and the like.
FIG. 1 is a circuit diagram showing a discharge circuit used for an excimer (excited dimer) laser device as a conventional discharge excited laser device. In the figure, reference numeral 10 designates a high voltage power source; 11 a charge reactor connected to one end of the high voltage power source 10; 2 a charge capacitor connected in series to the reactor 11; 9 a pair of discharge electrodes connected between the charge capacitor 2 and the other end of the high voltage power source 10; 13 a laser beam resulting from the discharge between the paired electrodes 9; 12 a charge reactor connected in parallel to the paired electrodes 9; 8 a capacitor connected in parallel to the paired electrodes 9 and receiving electric charges from the capacitor 2; and 1 a thyratron, serving as a high voltage switch, connected between the other end of the high voltage power source 10 and a node between the reactor 11 and the capacitor 2. The pair of electrodes 9 are disposed in discharge gas, such as rare gas and halogen gas.
FIG. 2 is a perspective view showing an excimer laser device using the discharge circuit of FIG. 1. In the figure, like reference symbols are used for designating like or equivalent portions in FIG. 1. In the illustration of FIG. 2, the high voltage power source 10, and the reactors 11 and 12, which are shown in FIG. 1, are omitted, for simplicity.
In FIG. 2, the thyratron 1 as a high voltage switch is contained in a tubular container 6 made of conductive material. One end of the container 6 is closed by a cover 7 made of insulating material.
A plurality of charge capacitors 2 are arrayed in a line. One end of the line of the capacitors 2 is connected to one end of the thyratron 1 by means of an L-shaped conductive plate 4. The other end of the line of the capacitors 2 is connected to an elongated conductive plate 3. A parallelepiped case 5 made of conductive material is provided. The case 5 is filled with discharge gas. A pair of electrodes 9 are disposed in the case 5. One of the paired electrodes 9 is mounted on the conductive plate 3, while the other is mounted on the bottom of the case 5. One end of the case 5 is connected to the other end of the thyratron 1 through the container 6. The plurality of capacitors 8 are provided between one of the inner side walls of the case 5 and the side wall of the conductive plate 3, both the side walls facing each other. Similarly, the plurality of capacitors 8 are provided between the other of the inner side walls of the case 5 and the side wall of the conductive plate 3, both the side walls facing each other.
The assembly thus constructed may be electrically expressed by an equivalent circuit, which is substantially the same as that of FIG. 1.
The operation of the discharge circuit thus constructed will be described.
In FIG. 1, when the high voltage power source 10 is turned on while the thyratron 1 is in an off state, a charge loop of the high voltage power source 10 - reactor 11 - capacitor 2 - reactor 12 - high voltage power source 10 is formed to charge the capacitor 2. Then, when the thyratron 1 is fired, a charge transfer loop of capacitor 2 - thyratron 1 - capacitor 8 is formed. Through the charge transfer loop, the capacitor 2 is discharged and the charge discharged is transferred to the capacitor 8. Accordingly, the voltage across the capacitor 8, i.e., the voltage between the paired electrodes 9, rises. When it reaches the discharge voltage, discharge occurs between the paired electrodes 9. The discharge excites the gas in the case 5 to emit a pulsative laser beam 13.
The conventional discharge circuit employs the thyratron 1 as the high voltage switch. However, the thyratron 1 has a large inductance, and therefore the voltage between the electrodes 9 slowly rises. As a result, the discharge is instable. Further, since the thyratron requires a preheating time and is sensitive to temperature, a temperature controller must be provided.
To cope with this, a switch consisting of a number of high speed switch elements such as FETs, IGBTs or thyristors, as a switch to be used in place of the thyratron, was proposed by the applicants of the present application. FIG. 3 is a schematic illustration of a switch device 15 for a laser device consisting of a number of high speed switch elements 14 in which series-connected elements are further connected in parallel. By simultaneously turning on all the high speed switch elements 14 making up the switch device 15 which is used in place of the thyratron 1 in FIG. 1, the discharge current is allowed to flow from the capacitor 2.
Since the switch device 15 using the high speed switch elements 14 is thus constructed, when it is applied to the discharge exciting pulse laser device such as the excimer laser device, a switching speed of the discharge circuit influences an exciting efficiency of the laser medium. In order to obtain a higher exciting efficiency, a high speed switch must be used for the discharge circuit. However, in general, high speed switches have small rated voltage and current. Thus, such switches, if used, must be connected in a series-parallel fashion.
In a switch consisting of series-parallel connected high speed switches, if the number of series-parallel elements is large, the switch suffers from the following problems:
a) The inductance associated with the wires is increased.
b) Reliability of the wires deteriorates.
c) Intricate work is required for wiring and maintenance.
In a case where MOSFETs (900 V and 15 A, as rated values), suitable for high speed operation, are used as the switch elements 14 in the discharge circuit of the excimer laser device, and the switch consisting of the MOSFETs must withstand 30 kV and 4000 A, (solid-state switch) elements 14 totaling 9078 (=34.times.267) must be series-parallel connected. Accordingly, the above problems become actualized.
Particularly when the inductance in the circuit increases, the switching speed is made slow, and the efficiency of the charge transfer from the capacitor 2 to the capacitor 8 is reduced. This results in reduction of the laser oscillation efficiency. For this reason, the circuit inductance must be minimized.
Further, in the laser switch device as shown in FIG. 3, if some of the high speed switches connected in series, parallel or series-parallel connected break down to be shortcircuited, the voltage applied to the normal switches is increased. Thus, even if the number of the series and/or parallel connections is selected so that each high speed switch has the rated voltage larger than the required voltage, when some switches break down, a problem arises that a voltage larger than the rated voltage is applied to the normal switches, and a steep pulse voltage destroys the high speed switch if the laser device is continued to run leaving the switch trouble unrepaired. Also, in a situation where some of the series and/or parallel connected high speed switches are shortcircuited, but the remaining ones are normally operating, an operator may fail to find the switch trouble. This is a very dangerous situation.
To realize the switch device 15, a plurality of FETs 14 (in the following description, the high speed switch element 14 will be referred to as FETs 14) are interconnected into a single module. A plurality of such modules are connected into the switch device 15.
FIG. 4 is a perspective view showing an excimer laser device using the switch device 15 consisting of the modules, which is to be used in place of the thyratron 1. In the figure, like reference symbols are used for designating like or equivalent portions in FIG. 2.
In FIG. 4, reference numeral 20 designates the module with a terminal plate 20a. A set of modules 20 are mounted on each of conductive plates 617. The conductive plates 617 are interconnected through the terminal plates 20a of the modules 20. The conductive plates 617 are placed on insulating members 619, which are layered on both sides of conductive plate 618 with a T-shaped top. The terminal plates 20a of the set of the modules 20 mounted on the uppermost conductive plates 617 are connected to the T-shaped top of the conductive plate 618. In this instance, although the lowermost conductive plates 617 are integral with a case 5, those conductive plates 617 may be formed separately from the case 5. The bottom end of the conductive plate 618 is connected to one end of a plurality of capacitors 2.
The operation of the laser device thus constructed will be described.
Voltage is first applied from the high voltage power source 10 to the top of the conductive plate 618. The capacitors 2 are charged through the conductive plate 618. Then, all of the FETs 14 contained in all of the modules 20 are simultaneously turned on, to form a charge transfer loop of the capacitor 2 - switch device 15 - case 5 - capacitor 8. Thus, the charge is transferred from the capacitors 2 to the capacitors 8. Consequently, the voltage between the paired electrodes 9 rises and a discharge starts.
In the laser device under discussion, all of the FETs 14 are turned on at a high speed. The voltage between the paired electrodes 9 sharply rises, providing a stable discharge.
However, since the switch device consisting of the FET modules, used for the laser device of FIG. 4, is thus constructed, the heat due to the operation of a number of FETs 14 fabricated into modules, may damage the FETs 14 per se.
FIG. 5 is a cross sectional view showing the module 20. The module 20 contains a circuit in which a plurality of FETs 14 are simultaneously turned on. A drain electrode 190, common to those FETs 14, is provided on the upper surface of the module 20. A source electrode 200 common to the FETs and a gate electrode 210 are provided on the lower surface of the module 20. The drain electrode 190 is shaped in a thin plate, and the source electrode 200 and the gate electrode 210 are shaped in line pin terminals and protruded from the side edge of the module 20 outwardly.
The module 20 constituting the switch device 15, which consists of a number of solid-state switch elements such as FETs, is constructed as shown in FIG. 5. As shown, the source electrode 200 and the gate electrode 210 are protruded from the lower surface of the module 20. One approach to construct the switch device 15 using a number of modules 20, is to arrange the modules 20 two dimensionally as shown in FIG. 4. Another approach is to arrange the modules 20 into a stack. Particularly in the approach to stacking the modules, the protrusion of the pin-terminal like electrodes from the lower surface of the module 20 makes it difficult to stack the modules and to save the space. Accordingly, it is difficult to make the switch device miniatured and compact. Additionally, the inductance of the circuit is increased.
FIG. 6 is a perspective view showing the combination of the module 1116 and a controlling unit for control the module. The module 1116 contains a plurality of FETs. The control unit 1117 for driving and controlling the module is provided on a substrate board 1117a. The control unit 1117 applies control signals to the module 1116 through lead wires 1118.
FIG. 7 is a perspective view showing an excimer laser device using the switch device 15 consisting of the modules 20, in place of the thyratron 1. In the figure, like reference symbols are used for designating like or equivalent portions in FIG. 6.
In FIG. 7, a number of modules 1116 are grouped into sets of modules each consisting a predetermined number of modules. Each set of modules is provided on each conductive plate 1119 which is disposed on a side of a conductive plate 1120. The lower end of the conductive plates 1120 is connected to one end of each capacitor 2. A container 1121, accommodating the case 5 and the like, has a hole 21a through which a laser beam 13 passes to exterior.
The operation of the laser device thus constructed is the same as that of the device shown in FIG. 4.
In the switch device 15 consisting of the modules each containing a plurality of solid-state switch elements constructed as shown in FIG. 6, the control unit 1117 and the module 1116 are connected by means of the lead wires 1118. With such a connection, the control unit 1117 is suspended from the module 1116 by the lead wires 1118, and a number of connection points are required. Therefore, failure frequently occurs. Further, if the lengths of the lead wires 1118 are unequal, the drive times are not uniform, deteriorating the switching performance. If the nonuniformity of the drive times is great, overvoltage is applied on a number of solid-state switch elements, possibly destroying the elements.
FIG. 8 is a block diagram showing a conventional control circuit 211 for controlling the high speed solid-state switch elements 14. The control circuit 211 is constructed such that a trigger circuit 213 is provided on a substrate 212, and the trigger circuit applies a trigger signal to the switch elements 14 through signal lines 214 to 217 formed on the substrate 212, and through trigger terminals 218 of the switch elements 14.
The thus constructed switch device 15 consisting of the plurality of switch elements 14 has the problems on the switching operation as follows.
Those problems will be described using a case where the switch device 15 is applied to the excimer laser device. In the following description, a "switch time" is defined as the time which is required for the voltage across the capacitor 2 in FIG. 1 to fall from 90 to 10% of a predetermined voltage. The rise time of a high voltage applied to the pair of electrodes 9 in FIG. 1 is within 100 to 300 ns. The switch time of the switch device 15 that is required at that time, must be 50 ns or less. In a case where the switch device 15 consists of a number of high speed switch elements 14, if those switch elements 14 are asynchronously switched, many problems arise. For example, the switch time of the whole switch device 15 is elongated, an overcurrent flows into the switch element 14 switched earlier than the other switch elements, and an overvoltage is applied to the switch element 14 switched later than the others. In the latter two cases, the switch elements are damaged or reduced in lifetime. Preferably, it is necessary to synchronize the switching operations of the switch elements 14, so that before the voltage assigned to one switch element falls to 20% of its initial value after the switch elements 14 are turned on, the remaining switch elements 14 are turned on. That is, it is necessary that the synchronization is achieved in terms of time within 10 ns or less.
The control circuit 211 of FIG. 8 is designed such that one trigger circuit 213 applies a trigger signal to the respective switch elements 14. Therefore, the signal lines 214-217 are not equal in length to each other. As a result, the switch elements 14 are driven at different time points. That is, the switching operations of those elements cannot be synchronized within a predetermined time. Thus, a jitter occurs in the fall of the voltage as shown in FIG. 9, and the switch time exceeds a predetermined switch time (for example, 50 ns).
In the series-parallel circuit of the switch device 15 shown in FIG. 3, when coincidence among the conduction timings of the parallel circuits is lost, overvoltage is applied to those parallel circuits later conductive, possibly destroying the high speed solid-state switch elements 14 such as FETs. To cope with this, protecting circuits 316 shown in FIGS. 10 and 11 were proposed by the applicants of the present Patent Application.
The protecting circuit 316 shown in FIG. 10 is constructed such that a reverse-current blocking diode 317 and a parallel circuit consisting of a capacitor 318 and a resistor 319 are connected in series. The protecting circuit 316 is connected in parallel to each parallel circuit 320 consisting of the switch elements (FETs) 14.
In the protecting circuit 316 thus constructed, when an overvoltage is applied to the parallel circuit 320 and the current flows through the parallel circuit, the capacitor 318 is gradually charged through the diode 317 according to a time constant determined by the capacitor 318 and the resistor 319. Accordingly, the overvoltage is not directly applied to the switch elements 14, thereby protecting the switch elements.
In the protecting circuit 316' shown in FIG. 11, a reverse current blocking diode 321 and an FET 322 are connected in series. A series circuit consisting of a Zener diode 323, a reverse current block diode 324, and a resistor 325, is branched from the cathode of the reverse current blocking diode 321. A node between the reverse current block diode 324 and the resistor 325 is connected to the gate terminal of an FET 322.
In the protecting circuit, when an overvoltage is applied to the parallel circuit 320', the Zener diode 323 becomes conductive through the reverse current blocking diode 321 and voltage appears across the resistor 325. This voltage is applied to the gate terminal of the FET 322, which in turn is conductive. As a result, the current caused by the overvoltage flows through the diode 321 and the FET 322, thereby protecting the switch elements (FETs) 14.
As seen from the foregoing description, the protecting circuits 316 of FIGS. 10 and 11 are both by-paths for the current caused by the overvoltage.
Since the switch device 15 with the conventional protecting circuit 316, 316' is thus constructed, if the switch device 15 is applied to the discharge circuit as shown in FIG. 1, the following problems arise.
When, in each of the parallel circuits 320, 320' shown in FIGS. 10 and 11, a power source (not shown) is provided on the left side on the drawings of those figures, inductance L of the conductor constituting a current loop for the current I at a location close to the power source is smaller than that at a location distant from the power source. FIG. 12 is a diagram useful in explaining the reason for this. In the figure, an area of the current loop of the current I is A and its depth is "l". In an actual circuit, the parallel circuits 320, 320' are arrayed in parallel while extending in the direction "l". Accordingly, "l" indicates the length of the array, and is a fixed length in the description to follow.
In FIG. 12, inductance L of the conductor forming the current loop I is given EQU L=.mu..sub.o .times.(A/l) (1)
where .mu..sub.o : dielectric constant.
The equation (1) indicates that in FIG. 12, the inductance L becomes larger toward the right side of the current loop I. The cross sectional area enclosed by the current loop I is referred to as the "inner side of the current loop". Then, inductance L of a subcurrent-loop within a main current loop is smaller than that outside the main current loop. To be more specific, in each of the circuits of FIGS. 10 and 11, with regard to the switch elements (FETs) 14 making up the parallel circuit 320, 320', the inductance L of the current loop for the switch elements (FET) 14 on the left side is small, and that for the switch elements (FET) 14 on the right side is large. The current I depends on the inductance L as follows: ##EQU1## where
Vo: Voltage
C: Synthesized capacitance of capacitors 2 and 8
t: Time.
FIG. 13 is a graph showing waveforms of the current I for two different inductances L and L' (L'&lt;L As seen from the graph, in the case of the small inductance L', the current waveform is sharp within a short time.
As seen from the foregoing description, in the circuits of FIGS. 10 and 11, even if the protecting circuit 316 is used, an overvoltage is earlier applied to the switch elements (FETs) 14 on the left side, viz., in the inner side of the current loop than the protecting circuit 316, 316'. As a result, the switch elements (FETs) will be destroyed.
FIGS. 14(A) and 14(B) are circuit diagrams showing a conventional pulse generating circuit for a discharge excited pulse laser as disclosed in Japanese Patent Application No. Heisei 2-34251. In the figures, reference numeral 42 designates a charge reactor; 43 a charge diode; 44 a main capacitor for charge and discharge; 45 a charge resistor; 46 a peaking capacitor; and 47 a discharge tube (laser tube) for generating a laser output by heating and evaporating metal (e.g., copper) contained therein through the gas discharge. A switch 415 for pulse generation consists of a number of FETs (field effect transistors) 49 as solid-state switch elements. Those FETs are connected in parallel to one another, and those parallel connections are connected in series. One specific FET 49A (also called an overvoltage protecting FET) of those FETs 49 of each parallel connection is connected such that the gate of the FET 49A is not connected to the gates of the other FETs 49.
A series connection of a Zener diode 411 and a diode 412 is connected between the gate and drain of the overvoltage protecting FET 49A. A resistor 413 is connected between the gate and source of the FET 49A. The Zener diode 411, the diode 412, and the resistor 413 make up a dynamic clamper. A diode 420, coupled with the drain of the FET 49A, is a reverse-current blocking diode.
When a gate signal is applied to the series-parallel connected FETs 49, the FETs 49 are turned on. A large charge voltage across the main capacitor 44 is applied through those FETs 49 to the discharge tube 47, feeding the discharge current thereto. If the coincidence among the switch timings of the FETs 49 is lost in an FET series connection, for example, a switch time t.sub.1 of one of the FETs 49 in the first series-connection stage S.sub.1 among those stages S.sub.1 to Sn is behind the switch time t.sub.0 of each of the FETs 49 of the remaining series-connection stages (FIG. 15(a)), a large voltage is concentrically applied across the source-drain paths of the FETs in the first series-connection stage (FIG. 15(b)). Under this condition, the FETs 49 in the first series-connection stage will possibly be damaged. When the overvoltage in excess of the Zener voltage Vb shown in FIG. 16 is applied to the Zener diode 411 connected between the drain and gate of the FET 49A, which is connected across the FET 49 in the first series-connection stage, since the diode cannot withstand the voltage exceeding the voltage Vb, current flows through the diode 412 to the resistor 413. The current i.sub.b flowing into the resistor 413 is as shown in FIG. 15(d). Accordingly, the voltage across the resistor 413 rises and the gate voltage also rises (FIG. 15(c)). When the gate voltage exceeds the threshold voltage, the drain-source path of the FET 49A immediately becomes conductive. Under this condition, the current, which otherwise is to be shunted into the respective FETs of the first series-connection stage, concentrically flows through the drain-source path of the FET 49A. Meanwhile, stray inductance 421 is parasitic on the source of the FET 49, and parallel stray inductance 422 is parasitic on the source of the FET 49A, as shown in FIG. 14(b). In the FIG. 14 circuit, the arrangement of circuit components are usually laid out so that the stray inductance 422 has a small value. If not so laid out, the rise of the current i.sub.b flowing through the resistor 413 becomes dull, and the rise of the gate voltage of the FET 49 becomes gentle. The conduction of the FET 49A retards, making its protecting function ineffectual. It is for this reason that the stray inductance 422 is selected to be smaller than the stray inductance 421. Accordingly, immediately after the FET is switched, the reverse current will flow into the FET 49A of the lower inductance by the electric energy stored in the stray inductances 421 and 422. The reverse-current blocking diode 420 blocks the rush of the reverse current into the FET 49A, thereby to protecting the FET 49A from being destroyed by the reverse current.
In the pulse generating circuit thus constructed, if the conduction timings of the series-parallel connected FETs are staggered, the FET (e.g., FET 49A) in the bank in which the conduction timing is behind may be destroyed by an overvoltage applied thereto. To cope with this, a dynamic clamper is connected in series to each FET, thereby to prevent the overvoltage being applied to the FET. However, since the overvoltage destruction of the FET is due to instantaneous electric energy, even the response delay of the Zener diode cannot be neglected. A slight response delay of the Zener diode, when combined with bad conditions, possibly leads to the overvoltage destruction.
FIGS. 17 and 18 are a side view and a perspective view showing a conventional discharge excited laser device as disclosed in Japanese Patent Application No. Heisei 2-163228. In the figure, reference numerals 1A and 1B indicate solid-state switch elements of 500 ns or less in switching time. Radiating plates 2A and 2B are provided for cooling the solid-state switching elements 1A and 1B. Conductive plates 3A, 3B, 60, 7A and 7B allow large current to pass therethrough. Numeral 2 designates a charge capacitor; 5 a case; 8 a peaking capacitor; and 9 discharge electrodes. FIG. 19 is an electric circuit showing the discharge portion in FIGS. 17 and 18. In the figure, reference numeral 10 designates a high voltage power source; and 11 and 12 charge reactors.
The operation of the discharge circuit shown in FIG. 19 is the same as that of the discharge circuit shown in FIG. 1 in which the thyratron 1 is used for the solid-state switch elements 1A and 1B. In the circuit shown in FIG. 19, the solid-state switching elements 1A and 1B are stacked in a multiple of stages, in order to obtain a high breakdown voltage. A number of banks of the switching elements are arrayed in parallel in the optical axis, in order to obtain a large current capacity. As viewed in the direction orthogonal to the optical axis, the switch groups are disposed on both sides of a phantom line passing through the paired discharge electrodes. Two discharge energy transfer loops are formed. One is a counterclockwise loop of charge capacitor 2 - conductive plate 60 - conductive plate 7A - solid-state switching element 1A - conductive plate 3A - conductive plate 60 - charge capacitor 2. The other is a clockwise loop of charge capacitor 2 - conductive plate 60 - conductive plate 7B - solid-state switching element 1B - conductive plate 3B - conductive plate 60 - charge capacitor 2. The direction of current flow of one loop is opposite to that of the other loop, so that the inductances caused by the currents are canceled out. Further, the switching elements 1A and 1B are disposed separately from each other in the optical axis direction. Such a layout of the switching elements successfully eliminates the phenomenon of the temporary current concentration, which may occur when the thyratron as a switch device is used. The result is a considerable reduction of the inductance of the charge transfer loop, and the consequent increase of dv/dt. Hence, a uniform and stable discharge can be obtained. For example, when the switch elements 1A and 1B of 40 ns in switching time is used, 100 nH or less of an overall inductance is attained.
In the solid-state switch device thus constructed, the portions of the switch device near the radiator plates is satisfactorily cooled, but the inner portion and some specific portions are unsatisfactorily cooled. As a result, a nonuniform temperature distribution appears in each of the solid-state switch elements (switches, such as FETs, IGBTs, and SITs, or other types of switches having a switching time comparable with that of the former switches) or in modules containing a plurality of switch elements. Accordingly, their switching characteristics are also not uniform, so that an overvoltage is applied to the switches turned on later. Extremely, those switches are destroyed.
FIG. 20 is a circuit diagram showing a conventional discharge excitated pulse laser device. FIG. 21 is a diagram showing waveforms for explaining the operation of the pulse laser device. In FIG. 20, reference numeral 141 designates a capacitor for storing energy for the main discharge; 142 a peaking capacitor; 143 a charge inductance; 144 a high voltage switch such as a thyratron for starting the discharge; 145 a first main electrode disposed within a laser medium 1410, the longitudinal direction of which coincides with the laser optical axis direction; 146 a second main electrode with a plurality of holes, which is disposed in opposition to and spaced by a predetermined distance from the first main electrode 145; 147 a dielectric material interposed between the second main electrode 146 and an auxiliary electrode 148 in a sandwich fashion; and 149 a high voltage generator. Letter A indicates a circuit used exclusively for a preparatory ionization. The circuit, provided for oscillation period adjustment, is made up of inductances 1411A, 1411B and a capacitor 1412 directly coupled with each other, for setting a time constant. Numeral 1413 represents a mid point between the time constant setting capacitor 1412 and the auxiliary electrode 148.
The operation of the discharge excited pulse laser device thus constructed will be described. The capacitor 141 is charged, with a high voltage, through the charge inductance 143, and the capacitor 1412 is charged, with a high voltage, through the charge inductances 1411A and 1411B. The charge current path for the capacitor 141 is denoted as 141X, and that for the capacitor 1412, as 141Y. Thereafter, the high voltage switch 144 is turned on. Voltage across the peaking capacitor 142, viz., voltage between the second main electrode 146 and the auxiliary electrode 148, rises like voltage V in FIG. 21. Current having a waveform denoted as I.sub.A in FIG. 21, which is for charging the peaking capacitor 142 and a capacitance formed of the second main electrode 146 and the auxiliary electrode 148, flows from the capacitor 141. A rate of the flow of the current 142X is determined by a combined capacitance of the capacitors 141 and 142, and the capacitor formed of the second main electrode 146 and the auxiliary electrode 148, and a stray inductance in the circuit, and rises within 50 to 100 ns, as current I.sub.B in FIG. 21. At the same time, the circuit, or the preparatory ionization dedicated circuit A, which is made up of the inductances 1411A and 1411B, and the capacitor 1412, which set a time constant, the auxiliary electrode 148, the dielectric material 147, the second main electrode 146, and the high voltage switch 144. The charge, which has been stored in the time constant setting capacitor 1412 and the capacitor constructed with the second main electrode 146, the dielectric material 147, and the auxiliary electrode 148, is discharged therefrom through a path 142Y. Through the discharge, a creeping discharge occurs at the plurality of holes of the second main electrode 146, so that a preparatory discharge is performed.
Assuming that the capacitance of the preparatory ionization dedicated circuit A is C, and the sum of the inductances of the same circuit is L, then the oscillation period is 2.pi..sqroot.LC.
Generally, a quantity of charge necessary for the preparatory ionization is approximately 1% of the quantity of charge for the main discharge. Therefore, by setting the C to be small, the oscillation period of the preparatory ionization dedicated circuit A may be 1/10 or less of the oscillation period of the main discharge circuit. Waveforms of the operation as stated above is shown in FIG. 21. As shown, until the main discharge occurs, current I.sub.B of the preparatory ionization oscillates two to three times, improving the uniformity of the preparatory ionization. Thus, the transfer of the main discharge to the arc is suppressed and hence uniform main discharge condition is set up.
Since the conventional discharge excited pulse laser device is thus constructed, if the thyratron is used for the high voltage switch for discharge start, the current of the discharge circuit concentrates at the electrodes to cause great inductance. The great inductance makes it difficult to switch at high speed. To cope with this, a measure has been taken in which the current of the main discharge circuit is temporarily restricted by a saturable reactor, for example, and the preparatory discharge is performed during the current restriction period. The measure makes the circuit construction complicated.
FIG. 22 is a circuit diagram showing a basic pulse generating circuit used for a conventional laser oscillator that is disclosed in IEEE Journal of Quantum Electronics, Vol. QE-15, No. 5, May, 1979, PP311-313. The pulse generating circuit includes a pulse circuit called an LC inverting circuit. In the figure, reference numeral 161 designates a thyratron used as a high voltage switch; 162 a first capacitor C.sub.1 ; 163 a second capacitor C.sub.2 ; 164 a high voltage power source; 165 a charge resistor; and 166 a discharge section.
In operation, the high voltage power source 164 feeds charge current into the following two charge loops; a first loop containing inductive element L, a diode D, the second capacitor 163, and the charge resistor 165; and a second loop including the inductive element L, the diode D, and the first capacitor 162. As the result of the current flow, charge is stored in both the capacitors. At this time, the voltage at point A is zero. Then, the thyratron 161 as a high voltage switch is closed, and the discharge is performed through a loop including the first capacitor 162 and the thyratron 161. As a result, the voltage at point A rises to be doubled. With progress of the above operation, the voltage between the main electrodes in the discharge section 166 rapidly rises and eventually a discharge space is broken down. The energy stored in the capacitor is fed into the discharge section 166. The gas in the discharge space is excited to emit a laser beam by the stimulated emission. In the LC inverting circuit, the charge voltage may be lower than the voltage necessary for the discharge section 166. Therefore, the switch of a low withstanding voltage may be used for the high voltage switch. Additionally, the size of the high voltage power source 164 and the like may be reduced.
FIG. 23 is a circuit diagram showing a discharge section of a conventional discharge excited laser device. This circuit additionally includes a magnetic saturation switch 167 and a peaking capacitor 168. In this circuit, the current flowing into the discharge section 166 is temporarily shut off by utilizing the characteristic of the magnetic saturation switch 167. As far as the magnetic member of the magnetic saturation switch 167 is not saturated, its impedance .omega.L is extremely high. When the magnetic member is saturated, the impedance is abruptly decreased. For example, when the thyratron 161 is turned on, the impedance .omega.L of the magnetic saturation switch 167 is large, so that the rise of the discharge current is delayed. When the magnetic saturation switch 167 is saturated, charge is transferred to the peaking capacitor 168, and its charge applies a high voltage to the discharge section 166 to cause it to start a discharge.
The conventional discharge excited pulse laser device is thus arranged. Accordingly, a pulse (high frequency) circuit including the inductive element is formed in the charge transfer loop including the thyratron, so that a response time of the loop is long. Thus, a charging time for voltage inversion in the loop including the first capacitor 162 and the thyratron 161 is long. As a result, the discharge is impelled to start before the voltage at point A is doubled. Accordingly, the charge fed into the discharge section is insufficient in amount, and the resultant oscillation efficiency of the laser device is unsatisfactory.
The magnetic saturation switch, when used, is advantageous in that the voltage at point A is doubled and the load for the thyratron is small. However, since the magnetic saturation switch is an active element, when the charge voltage is varied or ambient temperature is varied, the switching time also varies. In this respect, difficulty exists in using this type of the switch.
FIG. 24 is a circuit diagram showing a conventional exciting circuit for an excimer laser as discussed in "Electrical Society Technical Report (Part II), No. 217 (Present State of Shortwave Laser), page 5, issued April, 1986 (Showa 61). In the figure, reference numerals 171 and 172 designate a pair of main discharge electrodes; 173 a peaking capacitor connected in parallel to the main discharge electrodes 171 and 172; and 174 a pulse generating capacitor connected at one end to the main discharge electrode 171. A switch 175, for which a thyratron was used, is connected between the other end of the pulse generating capacitor 174 and the main discharge electrode 172. Numeral 176 represents a charge reactor connected in parallel to the peaking capacitor 173, and numeral 177 indicates a charge terminal.
The operation of the excimer laser exciting circuit thus arranged will be described. When a high positive voltage is applied to the charge terminal 177, a charge current i.sub.1 flows through the charge reactor 176 to charge the pulse generating capacitor 174 with the polarities as shown. Under this condition, when the switch 175 is turned on at time point t.sub.0, the voltages across the pulse generating capacitor 174 and the peaking capacitor 173 vary as shown in FIGS. 25(a) and 25(b). The pulse generating capacitor 174 at charge voltage V.sub.1 starts to discharge, and current i.sub.2 flows through the peaking capacitor 173, to perform the charge transfer.
During a period between t.sub.0 .ltoreq.t.ltoreq.t.sub.1, the charge stored in the pulse generating capacitor 174 is transferred to the peaking capacitor 173. At time point t=t.sub.1, the discharge starts between the main discharge electrodes 171 and 172, and current i.sub.3 flows in the direction of arrow. In the excimer laser, actually, a preparatory ionizing discharge is performed prior to the main discharge. The electrodes and circuit for the preparatory ionizing discharge are omitted in FIG. 24 for simplicity. During a period between t.sub.1 .ltoreq.t.ltoreq.t.sub.2, when energy is injected into the discharge section from the peaking capacitor 173, the main discharge is performed between the main discharge electrodes 171 and 172, to cause a laser oscillation. In the laser of a small discharge resistance (e.g., 0.2 .OMEGA.), such as an excimer laser, the voltage V.sub.2 across the peaking capacitor 173 takes an oscillation voltage waveform as shown in FIG. 25(b), and hence a voltage of the reversed polarity is caused (t=t.sub.2 in FIG. 25(b)). At a time point where the oscillation almost terminates, a voltage having the polarity reverse to that of the voltage at the time of the charging, appears across the pulse generating capacitor 174 (Vr in FIG. 25(a)). The voltage Vr causes the reverse current i.sub.4 to flow.
FIG. 26 is a circuit diagram showing another conventional exciting circuit for a pulse laser. In this circuit, the charge in the pulse generating capacitor 174 flows in the form of a charge current i.sub.20 through a saturable reactor 178, which acts like a switch, and transfers to the peaking capacitor 173. This circuit, generally called an MPC (magnetic pulse compression) circuit, includes additionally another pulse generating capacitor 179 and a current restricting reactor 1710, in order to reduce a switching loss at the switch 175. Operating waveforms of the signals in the MPC circuit are as shown in FIGS. 11(a), 11(b) and 11(c). As in the conventional circuit of FIG. 24, at time point t=t.sub.1, the discharge starts between the main discharge electrodes 171 and 172. Subsequently, the reverse current i.sub.4 oscillates. Finally, the voltage Vr having the polarity reverse to that of the voltage at the initial stage, appears across the pulse generating capacitor 174.
In the conventional pulse laser device thus constructed, the reverse voltage Vr of the reversed polarity appears across the pulse generating capacitor, after the main discharge current flows. The reverse voltage energy (=1/2CVr.sup.2) is consumed in the form of arc or streamer between the main discharge electrodes after a relatively long time (e.g., approximately 1 .mu.s) from the main discharge occurrence. This is called an after-current, and does not contribute to the laser oscillation, but damages the main discharge electrodes to reduce the lifetime of the electrodes. Further, when the after-current flows, the laser device cannot oscillate at a high repetitive frequency.
FIG. 28 is a circuit diagram showing an electrical circuit of another conventional discharge excited pulse laser as disclosed in Electrical Society Technical Report (part II), No. 217 (Present State of Short Wave Laser), P5, issued April, 1986 (Showa 61). In the figure, reference numerals 181 and 182 designate paired main discharge electrodes disposed confronting each other; numeral 183 a peaking capacitor C.sub.2 made of SrTiO.sub.3 (titanic acid strontium) and disposed in parallel to the main discharge electrodes 181 and 182; numeral 184 a pulse generating capacitor C.sub.1, one terminal of which is connected to the main discharge electrode 181. A switch 185 of a thyratron is connected between the other terminal of the capacitor 184 and the main discharge electrode 182. A charge reactor 186 is disposed in parallel to the peaking capacitor 183. Reference numeral 187 designates a high voltage power source. Numerals 188 and 189 indicate inductance components contained in the circuit.
In operation, when a positive high voltage is applied from the high voltage power source 187 to the pulse generating capacitor 184, charge current i.sub.1 flows through the charge reactor 186 to charge the capacitor 184 with the polarity as shown. When the switch 185 is turned on, the charge of the capacitor (C.sub.1) 184 flows, in the form of pulse current i.sub.2, into the peaking capacitor (C.sub.2) 183. In this way, the charge transfer is performed. Succeedingly, a discharge starts between the main discharge electrodes 181 and 182, and discharge current i.sub.3 flows in the direction of an arrow. In the excimer laser device, a preparatory discharge is performed prior to the main discharge. The electrodes and electric circuit for the preparatory discharge are not illustrated here. When the energy is injected from the peaking capacitor (C.sub.2) 183 into the discharge section, the main discharge occurs between the paired main discharge electrodes 181 and 182, causing a laser oscillation. In the laser device of a low discharge resistance (e.g., 0.2 .OMEGA.), such as an excimer laser device, the voltage across peaking capacitor (C.sub.2 ) 183 varies as indicated by an oscillation voltage waveform shown in FIG. 30(a). As seen, the voltage component of the inverted polarity is observed. The voltage component Vr of the inverted polarity causes an inverted current (after-current) as shown in FIG. 30(b), which in turn causes an arcing. As a result, the lifetime of the main discharge electrodes 181 and 182, and the laser gas are remarkably reduced.
FIG. 29 is a graph showing a characteristic curve representing a variation of the capacitance of the peaking capacitor (C.sub.2) 183 vs. DC voltage, when the capacitor 183 is made of SrTiO.sub.3. The graph shows that the capacitance hardly changes even if the voltage is decreased up to a value near to the rated voltage. This shows that after the discharge starts, energy is left in the peaking capacitor (C.sub.2) 183, and the residual energy causes the arc discharge owing to the aftercurrent.
In the conventional discharge excited pulse laser device thus constructed, when the peaking capacitor (C.sub.2) 183 is made of SrTiO.sub.3, its capacitance hardly varies even if the voltage is increased up to approximately the rated voltage. Accordingly, after the discharge starts, energy is left in the peaking capacitor (C.sub.2) 183, and the residual energy causes the arc discharge owing to the after-current.
In the charge transfer type circuit, when a voltage higher than the voltage Vo applied from the high voltage power source, is applied to the discharge section, if a ratio of C.sub.1 /C.sub.2 is set large, the voltage applied to the discharge section can be increased up to a value two times the applied voltage Vo. However, also in this case, after the discharge starts, the amount of charge left in the peaking capacitor is large, so that arc discharge owing to the after-current tends to occur.